A light wave is plane-polarized or linearly polarized when all of the electric field vectors in the light wave perpendicular to the direction of wave travel lie in a given plane. The orientation of the given plane is the direction of polarization. For example, a horizontally polarized light wave has a vertical amplitude of zero, and a vertically polarized light wave has a horizontal amplitude of zero. An unpolarized light wave is one that propagates in more than one plane. Polarization of a light wave (e.g., an optical signal) can occur when a light wave is reflected off a non-metallic medium (e.g., a beam splitter), forming a reflected light wave and a refracted light wave. When polarization occurs via reflection (e.g., from a mirror), the extent to which the polarization occurs is dependent on the angle at which the light approaches the medium.
In many devices configured to receive and/or transmit an optical signal (e.g., a data, voice and/or video signal in an optical or optoelectronic network), polarization can play an important role in the effective transmission of an optical signal. For example, as shown in FIG. 1, a conventional optical receiver 100 comprises an optical fiber (not shown), a lens 110 (optional), a mirror 120, a filter 130, and a receiver 140. As shown, lens 110 receives a light signal IN (e.g., from the optical fiber) and provides a focused light signal 150 to mirror 120. Mirror 120 then reflects the light signal 150 to receiver 140 in the form of a reflected light signal 155 for further processing. The reflected light signal 155 may pass through filter 130 before being received in the receiver 140. Reflected light signal 155 is at least partially polarized (e.g., having electric field vectors in planes at certain angles with reduced amplitudes).
As shown, mirror 120 is positioned at a 45° angle (i.e., the angle of incidence) with respect to optical signal 150. When optical signal 150 travels from a first material (e.g., the air between lens 110 and mirror 120) having a first index of refraction (n1) to a second material (e.g., mirror 120) having a second index of refraction greater than the first index of refraction (n2), a portion of the optical signal 150 is refracted into the second material 120 and a portion of the optical signal 150 is reflected back into the first material (e.g., towards receiver 140). For optical signals reflected by non-metallic surfaces, if the angle of incidence is such that the reflected and the refracted rays are at the Brewster angle (i.e., tan−1 [n2/n1]), the reflected ray is linearly polarized parallel to the reflective surface. Thus, the intensity (I1) of the transmitted signal (e.g., the reflected optical signal 155) can be calculated according to Equation [1] below:I1=I cos2(θ)  [1]    where I is the intensity of the incident optical signal 150, and θ is the angle of incidence of optical signal 150. Thus, as θ approaches 90°, the value of cos2(θ) approaches zero such that the intensity of the reflected signal decreases and the degree of polarization increases (e.g., up to 100% polarization). Furthermore, placing a mirror at a 45° angle in an optical receiver requires precise positioning during the manufacturing process, and slight variations in the angle of the mirror can greatly affect the degree of polarization (and thus the amount of lost intensity) in the reflected light wave(s).
This “Background” section is provided for background information only. The statements in this “Background” are not an admission that the subject matter disclosed in this “Background” section constitutes prior art to the present disclosure, and no part of this “Background” section may be used as an admission that any part of this application, including this “Background” section, constitutes prior art to the present disclosure.